1. Technical Field
The invention is related to methods for forming ceramic matrix composites having ceramic fibers coated with a toughening layer which facilitates fiber de-bonding and pull-out in the wake of a crack in the matrix.
2. Background Art
Fiber reinforced ceramic matrix composites comprise a weave of ceramic fibers embedded in a ceramic matrix. One way of improving the mechanical properties of such a composition is to provide a coating over the fibers which is stable and resistant to oxidation and which promotes fiber de-bonding at the tip of an advancing crack and fiber bridging and eventually pull-out in the wake of an advancing crack in the composite. This feature enhances the toughness, strength and strain to failure of the composite because the fibers remain mostly immune to advancing cracks in the matrix. The history of development of this technique is described in U.S. Pat. No. 4,885,199 to Corbin et al.
For certain applications, the ceramic matrix composite must be stable at temperatures above 2200 degrees F. in an oxidizing environment. Well-known fiber coatings such as carbon and boron nitride are not stable under such conditions. Any material to be substituted for a fiber coating in place of the carbon or boron nitride must be both resistant to oxidation and must possess sufficient strength to transfer loads from the matrix to the fiber while having a low shear strength to promote debonding between the fiber and the matrix in the presence of an advancing crack. Furthermore, the coating must be easy to apply to macrofibers, fiber tows and to a weave of ceramic fibers during manufacturing.
U.S. Pat. No. 4,397,901 to Warren discloses a ceramic coating on a ceramic fiber to accommodate a thermal expansion mismatch. U.S. Pat. No. 4,935,387 and U.S. Pat. No. 4,948,758, both to Beall et al., disclose a sheet silicate coating on fibers which promotes fiber pull-out by cleavage failures between crystal sheets. The disclosures of Beall et al. rely upon the intrinsic nature of the crystalline cleavage or bond between the silicate sheets to promote fiber pull-out. U.S. Pat. No. 4,869,943 and U.S. Pat. No. 4,885,199, both to Corbin et al., disclose toughening a ceramic matrix with a fiber coating such as a pyrolytic carbon or other material which differs either in morphology or chemistry from the fiber and the matrix, thereby providing a crack deflection zone. U.S. Pat. No. 4,772,524 to Coblenz discloses a fibrous monolith, not a fiber/matrix composite, in which planes of weakness between adjacent fibers deflect advancing cracks in the monolith. U.S. Pat. No. 4,642,271 to Rice and U.S. Pat. No. 4,605,588 to Simpson et al. both disclose a BN coating on ceramic fibers. Rice discloses that the coated fibers are in a matrix and the fiber coating promotes fiber pull-out. U.S. Pat. No. 4,916,092 to Tiegs et al. discloses SiC whiskers with a layer of carbon in a whisker reinforced ceramic composite, in which none of the coatings are oxidation resistant. U.S. Pat. No. 4,543,345 to Wei discloses whisker reinforced composites with an alumina or mullite coating on the whiskers.
The prior art as described in U.S. Pat. No. 4,885,199 referenced above typically relied upon the characteristics of the inherently weak shear strength of the carbon and boron nitride coatings to achieve desired characteristics, such as toughening. For applications in the high temperature oxidizing environments described above, the intrinsic properties of the coating composition would have to provide all of the necessary features, including fiber de-bonding and pull-out as well as imperviousness to oxidation and high temperatures. The problem with this approach is that it is very difficult to select the best fiber coating material for a given ceramic fiber so as to optimize all of the foregoing features in the same coating material composition.
Thus, one object of the present invention is to depart from the prior art approach of finding a coating composition which provides all of the necessary features, and instead find a mechanical approach in which all, or at least some, of the desired features (such as fiber de-bonding and pull-out in the wake of an advancing crack in the matrix) are realized through the mechanical features of the coating and coating/fiber interface, as distinguished from the inherent features of the composition. Such a mechanical approach has many advantages and, in most cases, allows greater choice in selecting the materials for use in the coating. For example, the materials can be chosen to meet only the requirement of resistance to oxidation and stability at high temperatures, while the remaining requirements (e.g., fiber de-bonding and pull-out) are met by mechanical features in the coating or coating/fiber interface. In fact, the same composition as the fiber and/or matrix would be a candidate for the fiber coating. This advantage will become clear in the description of the invention which follows the conclusion of this description of the background art.
In the present invention, the mechanical feature which promotes the requisite tendencies (e.g., fiber de-bonding and pull-out in the wake of an advancing crack in the fiber/matrix composite) is the presence of empty pores in the fiber coating. A method of forming a porous alumina coating on a ceramic fiber which is then covered with a metal coating prior to immersion in a matrix is disclosed in U.S. Pat. No. 4,935,296 to Stevens. The Stevens patent is directed to metal-coated ceramic fibers in a matrix and in particular to solving the problem of interlocking the metal coating with an interlayer. The Stevens patent solves this problem by leaving the pores in the fiber coating open and permitting the metal coating to fill the open pores of the porous coating on the fiber, so that there is greater adhesion between the metal and the porous coating. This technique of the Stevens patent therefore achieves the opposite purpose intended in the present invention, which is not to increase adhesion (as in Stevens) but instead to permit de-bonding and fiber pull-out. In the Stevens patent, the matrix is not in contact with the porous fiber coating, but instead contacts the overlying metal coating.
Accordingly, it is an object of the invention to provide a method for forming a porous coating with empty, sealed pores on a ceramic fiber in a ceramic matrix so that the empty pores remain empty after incorporation of the fiber into a ceramic matrix and permit the fiber to de-bond and pull out from the coating under stress.
It is a further object of the invention to provide a ceramic fiber/ceramic matrix composite having a porous coating covering the fibers with empty pores which render the fiber coating frangible, thus promoting fiber de-bonding and pull-out from the coating in the wake of an advancing crack in the ceramic matrix.
It is a related object of the invention to provide a ceramic fiber/ceramic matrix composite having a sealing coating covering the fibers and directly contacting the ceramic matrix.